Active transconductance circuits are generally characterized by high linearity and high output impedance. These characteristics are desirable at the input of an active RC filter, for example, a radio receiver channel-select filter.
FIG. 1 diagrammatically illustrates an example of a conventional active RC low-pass filter. If the operational amplifier A is assumed to be ideal (infinite DC gain and bandwidth), then the filter response is entirely determined by the resistor ratio (α) and the RC product. However, a non-ideal operational amplifier will affect the filter response. For a variable gain implementation, the filter response even depends upon the gain setting.
FIG. 2 diagrammatically illustrates a conventional solution which reduces the non-ideal operational amplifier's impact on the filter response. In FIG. 2, the input resistance R/a of FIG. 1 is replaced by an active input transconductance a/R. The high output impedance of the transconductance eliminates the influence of the resistor R/a on the filter response (see also FIG. 1). Although the non-ideal operational amplifier does affect the filter response in FIG. 2, this is independent of the gain setting of the filter, and the effect is less significant than in the FIG. 1 filter.
Ever increasing cost reductions in integrated circuits require continued migration toward ever smaller sized processes, which operate at ever lower supply voltages. These low supply voltages mean that the circuits must be able to operate with limited headroom. Voltage clipping due to limited headroom is a primary cause of harmonic distortion in active filters.
Although conventional active transconductances have the aforementioned advantages of high linearity and high output impedance, they nevertheless tend to produce harmonic distortion in active filters, to the extent that their behavior is non-linear and the available headroom is limited. Conventional active transconductances are also generally rather complex circuits whose performance relies on good component matching.